The present invention is directed to components for particle accelerator assemblies, and more particularly, to liquid-target holder assemblies on particle accelerators assemblies used to produce selected radioisotopes.
Biologically active radiochemicals containing short-life isotopes have been utilized for medical research as well as for therapeutic and diagnostic procedures. The radiochemicals have achieved very good results. The radiochemicals are produced by synthesizing radioisotopes created by irradiating small targets with a particle beam from a cyclotron or other particle accelerator. The biologically active radiochemicals have relatively short useful lives, so the radiochemicals must be produced at or very close to the location in which they are to be used, such as a hospital or lab. Locally producing the radiochemicals has not been economically feasible or practical for most hospitals or labs.
The radioisotopes can be produced using accelerators that generate high-energy particle beams or low-energy particle beams. High-energy accelerators, such as the Ebco TR30, which produces a proton beam with a current up to 2.0 mA and energies variable between 15-30 MeV, are very large and expensive. Accordingly, it is often not economically feasible for hospitals and labs to have such equipment on site. Lower energy accelerators, such as the Ebco TR19 series which produce proton beams with currents between 100 xcexcA and 2.0 mA and energies variable between 12 MeV-19 MeV, are smaller and less expensive.
It is highly desirable to produce the radiochemicals, such as fluorodeoxyglucose (FDG), from liquid targets with low energy accelerators in an economical manner in sufficient volumes for use as needed by the hospital or lab. The low energy accelerators, however, are more susceptible to several factors that limit the ability to effectively and economically produce the necessary volumes of the radioisotopes for the synthesis of the radiochemicals. These factors include the cost of the liquid targets used to form the radiochemicals and the amount of the radiochemicals needed per dose for the medical research, therapeutic, or diagnostic procedure. In order to achieve high purity and high specific activity, the target material is made of separated isotopes, which are very expensive, and the target volume should be kept as small as possible to reduce the cost of production. The volume of the radiochemical typically needed per dose is typically very small, so large amounts of the radiochemicals do not need to be produced at once. In fact, producing large volumes of the radiochemicals can result in waste if the chemical is not all used within its short useful life.
Other factors effecting production of radioisotopes from liquid targets with low energy accelerators include the configuration of the holding assemblies that retain the liquid target during the irradiation process. The holding assemblies must withstand severe environments created during the irradiation process and also enable the production of contaminant-free radiochemicals. When the liquid target is irradiated, the proton beam quickly heats the liquid target and creates high pressure within the target holder. The target holder must be capable of withstanding the elevated pressures without rupturing and without removing too much energy from the proton beam. Conventional liquid target holders have a thin front window through which the proton beams must pass before hitting the liquid target. Thicker windows are desirable to withstand the pressures generated from heating the liquid, but the thicker windows provide more mass through which the proton beam must pass before reaching the target. Accordingly, the thicker windows absorb more beam energy, thereby decreasing the effectiveness of the proton beam. When a low energy beam is used, it is highly desirable to ensure that as much energy remains in the proton beam as possible by the time it reaches its liquid target to maximize the beam""s efficiency for irradiating the liquid target. So, while the strength of the thick window is desired, the resulting engergy decrease in the beam is not.
Another factor includes providing a liquid target that will fully absorb the remaining energy of the proton beam. As the proton beam is passed into the target holder and the liquid target, the liquid target must have a sufficient depth or thickness so as to fully absorb the particles from the beam. If the proton beam passed completely through the liquid target and the target holder, the particle beam could create a radioactive environment external to the holding assembly.
Another significant factor in forming the radioisotopes or radiochemicals is controlling the liquid target""s temperature during the irradiation process. When the proton beam bombards the liquid target, the temperature of the target quickly increases. Heat must be efficiently drawn from the liquid target to prevent boiling or formation of bubbles within the liquid target. It is noted that bubbles are voids, having substantially less mass through which the proton beam must pass. Thus, boiling the liquid target creates a very undesirable condition that can result in the beam passing through the liquid target and the target holding assembly, which could result in a radioactive environment around the particle accelerator.
The quantity of radioisotopes produced in a liquid target is very small (e.g., an isotope concentration in the target may be in the order of 1xc3x971012), so it is important that the target body not introduce contaminants into the target material. Such contaminants would reduce the quantity of the available useful radioisotopes, and hinder the subsequent chemical processes in incorporating the radioisotope into the desired radiochemical.
Removal of the heat generated in the target is a significant problem that limits the magnitude of the incoming beam""s current and hence, the production rate. Higher production rates are achieved if beams with higher currents can be used. Prior art target holders have been made of silver, which has a high thermal conductivity that allows heat to be quickly drawn from the liquid target. The silver target holders, however, often have impurities within the metal that can react with the proton beam to create a contaminant to the liquid target and the radiochemical formed in the target holder. At high currents, such as 40-50 xcexcA, the silver target holders are typically only usable for one or two runs to create radioisotopes such as Flourine-18 before being too contaminated for further use to maintain sufficiently pure radiochemicals. At lower currents, such as 20-25 xcexcA, the silver target holder can be used for a few runs before the holder must either be replaced or cleaned. Other materials with low thermal efficiencies but better resistance to contamination have not been suitable for use as liquid-target holding assemblies for low energy accelerators. Accordingly, the silver target holders are used despite this susceptibility to contamination because of silver""s extremely high thermal conductivity.
The present invention provides an assembly with a liquid-target holder that overcomes the above and other problems experienced in the prior art. One embodiment of the invention provides a particle accelerator assembly having a liquid-target holding assembly for retaining the liquid target in a selected orientation for irradiation by a particle beam projected along a beam axis. The liquid-target holding assembly includes a mounting portion coupled to the particle accelerator and configured to receive the particle beam moving along the beam axis. A liquid-target holder is connected to the mounting portion and has a holder body with a target cavity therein sized to contain the selected liquid target. The target cavity has a longitudinal axis oriented at an acute angle relative to the beam axis. The target cavity has a first depth along an axis perpendicular to the longitudinal axis of the target cavity, and has a projected depth along the beam axis greater than the cavity""s first depth. The holder body is a a chemically inert material unreactive with the selected liquid target, the resulting radioisotopes, or any reaction byproducts created upon irradiation of the liquid target. In one embodiment, the liquid-target holder is made of niobium, which is chemically inert with, inter alia, Oxygen 18, Oxygen-15, Nitrogen 13,Carbon 11, Flourine-18, and reaction by-products from the irradiation to create these radioisotopes.
In one embodiment, the mounting portion has a mounting surface oriented at an acute angle in the range of approximately 10xc2x0 to 50xc2x0, inclusive, relative to the beam axis. The liquid-target holder is removably attached to the mounting surface such that the target cavity""s longitudinal axis is oriented at an acute angle in the range of approximately 10xc2x0 to 50xc2x0 inclusive, relative to the beam axis. The liquid-target holder of one embodiment includes a front window removably connected to the holder body, and a rear window integrally connected to the holder body. A cooling block is connected to the holder and configured to support the rear window and to direct cooling fluid against the rear window, thereby drawing heat away from the rear window and facilitating cooling of the liquid target during the irradiation process.